The present invention relates generally to systems for providing tactile feedback in virtual reality environments, and more particularly to a system for providing tactile feedback in a virtual reality environment, in which the tactile feedback is in the form of vibrotactile stimulation.
A variety of systems that provide the user with the sense of touch in a virtual reality or synthetic environment have been developed. The article by Karun B. Shimoga, "A Survey of Perceptual Feedback Issues in Dexterous Telemanipulations: Part II. Finger Touch Feedback," IEEE Proc. 1993 Virtual Reality Annual Int'l Symposium, Seattle, Wash., Sept., 271-279, discusses several of the existing systems for providing tactile feedback in synthetic environments. Shimoga classifies the existing systems as: (1) Visual displays; (2) Pneumatic stimulation (air jets, air pockets and air rings); (3) Vibro-tactile stimulation (vibrating blunt pins and vibrating voice coils); (4) Electrotactile stimulation; and (5) Functional Neuromuscular Stimulation (FNS).
In visual display systems, the status of touch of the slave fingers is indicated by the appearance of an icon or by the display of slave finger tip forces. One approach displays the locations of two fingers and the boundaries of the object as separate icons, which permits the user to determine the relationship between the fingers and the object surface. In another approach, the object boundary icons are displayed at all times, while the finger tip icons appear only when the fingers touch the object. These display approaches require the user to rely on the display in determining contact with the object. This indirect tactile feedback is less desirable since it limits the user to relatively slow finger movement. In addition, a tactile feedback display device is useless to a visually-impaired individual, which therefore limits the applications that might incorporate such a tactile feedback device.
Three versions of the pneumatic stimulation approaches are depicted in FIGS. 8-10 of the present application. FIG. 8 depicts a pneumatic stimulation tactile feedback system. In this version, a 12.times.12 array of micro air jets force air against the ventral surfaces of the operator's fingers, which are enclosed in a position sensing glove. This system was used to transmit alphabet characters to visually-impaired individuals.
FIG. 9 depicts an air pocket system in which minute air pockets are placed under the fingers 92 facing the finger tip pulps. As in FIG. 8, the fingers are enclosed in a glove. These air pockets are then pressurized to signal the touch of the remote finger. The touch sensor signal from the remote slave activates a pressure regulator that varies the air pressure within the mini air pockets.
FIG. 10 shows an air ring pneumatic system. In this system, ring-like inflatable balloon actuators are placed on the operator's fingers. When the rings are pressurized, the resulting squeezing sensation on the fingers can be interpreted as the signal of touch. The fingers are enclosed in a glove.
In all tactile feedback systems, as the number of skin contacts per unit area decreases, so does the effectiveness of the touch feedback. At the other extreme, as the number of skin contacts per unit area increases beyond a point, no further useful information is received by the fingers. The threshold point is where the spacing between the two skin contacts is equal to the minimum distance between two points that the human skin can discriminate at that location. This is referred to as the two point discrimination ability of the human hand. A disadvantage with the air jet systems is that the fingers get numb and temporarily lose their tactile abilities and thus have inferior two point discrimination ability. Furthermore, all pneumatic tactile feedback systems have an inherently slow response time, which limits the operating bandwidth of these devices, and hence the types of signals that can be sent to the user.
Shimoga, ibid., discloses a vibro-tactile stimulation system (FIG. 11 ) which uses an array of thin blunt wires placed below each finger. The wires then are vibrated at a desired frequency and amplitude using a battery of electric solenoids. The resulting tickling sensation on the fingers can be perceived by the operator as the touch feedback signal. This system is very complex, noisy and consumes a relatively large amount of power. Such a device is not simple enough to be mass produced.
Patrick (see also Exos, Product Literature on Dexterous Hand Master and Grip Master, Exos, Inc., Lexington, Mass. (1991)), discusses the EXOS Touchmaster, which uses conventional voice coil technology. This vibro-tactile system uses vibrating voice coils placed below each finger. The voice coils utilize an internal moving electromagnet with a fixed external magnet. The voice coils transmit low amplitude high frequency vibrations onto the skin of the operator. Compared to the vibrating pin systems this system has reduced mechanical complexity, noise and power consumption. Furthermore, this device is relatively small, is portable and does not restrict the normal range of finger motion in comparison to the vibrating pin system. The operating frequency of this device is about 250 Hz, which is approximately the optimum frequency at which the skin of human fingers is highly sensitive. This higher bandwidth permits improved mean task error over visual feedback systems in teleoperation and interaction with virtual environments, however, this system is relatively complex and thus costly. Furthermore, this system is relatively rigid and does not conform easily to the contour of one's skin. An array of vibrating piezoelectric crystals has been used in lieu of voice coils. Such a system is rather complex, and consumes too much power to be a portable device, which limits its ability to be mass produced.
FIG. 12 depicts an electrotactile approach in which mini electrodes are attached to the operator's fingers and hand, which provide electrical pulses of appropriate width and frequency to the operator's skin to signal remote touch. The resulting tickling sensation will imply to the operator that the slave fingers are touching something.
Electrical pulses can be used to provide stimulation to astronauts wearing thick gloves. This approach has the disadvantage that some operators dislike feeling electrical signals. Furthermore, the pulse width and frequency of the stimulation are crucial to properly simulating touch. In addition, extreme care must be taken in choosing the locations for placing the extero-cutaneous electrodes.
Another approach uses invasive techniques that apply signals directly to the neuromuscular system. If appropriately done, the operator's brain will interpret these signals as the touch of his own fingers. This approach has the disadvantages that connection must be made to the neuromuscular system, and involves high liability risks. As in the above electrotactile system, extreme care must be taken in choosing the locations for placing the intramuscular electrodes.
Another existing system, called a Tactor (depicted in FIG. 13), uses an array of tactors, which are connected to the system by wires made of a Shape Metal Alloy (SMA). The SMA is a Nickel-Titanium alloy, which returns to a predetermined shape when heated. In this device 130, a current passing through a wire of this Ni--Ti alloy causes the wire to shrink, which then causes the tip to press against the skin. Levers are formed from a sheet of BeCu. Each lever is bent up to protrude through a hole in the touch plate. An SMA wire is attached near the end of each lever and angles upward to a connector block. Electrical leads carry current from the computer (not shown) through the SMA wires. The BeCu base 136 makes a common electrical connection to all SMA wires. Electrical heating of the SMA wire causes the wire to contract, driving the tactor upward through the touch plate, where it contacts the finger. This device operates at about a 10 Hz response time. The most sensitive response of the human skin occurs at about 200 Hz, while additional sensation occurs until about 250 Hz. Thus, this device cannot provide a sufficient bandwidth range of operation.
Shimoga, ibid., has identified several requirements for those systems that apply forces to the human fingers. When human fingers are exposed to constant forces over sustained periods, the ability of the fingers to sense magnitudes and directions of the forces being applied temporarily deteriorates. Furthermore, human fingers cannot sense forces below 0.2N/cm.sup.2. Yet, human fingers also get fatigued when exposed to large pressures for prolonged periods. Thus, the contact areas must be designed such that the nominal pressure on the skin lies above the threshold pressure of 0.2N/cm.sup.2. Several requirements are necessary in a tactile feedback system in the form of an instrumented glove. First, the glove must be lightweight to reduce operator fatigue and to increase portability. Second, the glove must be compact and must not limit the natural motion ranges of the human fingers. Third, the glove must be precise in its measurements and its perceptual feedback. Fourth, the glove must be inexpensive. Fifth, the glove must avoid the use of sensory substitution, i.e., the system should be such that the operator does not have to think to understand the meaning of the signal. Finally, the system should be safe to use for extended periods of time and should be compatible with the dimensions and the motion ranges of the human hand. In addition, certain areas of the human hand that play an important role in grasping and manipulation have been identified. FIG. 14 depicts the zones of the palm and fingers of the human hand, identified by Shimoga, ibid., that are functional during grasping and manipulation tasks. The important areas are the distal phalanges of the thumb and the fingers as well as the central palmer region on the ventral side of the hand. Thus, a tactile feedback system need only cover these areas alone.
The requirements for the number of skin contacts that must be placed over a unit area so that the operator's fingers get adequate sensation of touch, i.e., the two point discrimination ability, have been identified. The index finger pulp can sense all points that are over 2 mm apart, while the center of the palm cannot discriminate two points that are less than 11 mm apart. Therefore, the optimum number of skin contacts per unit area at a given location on the hand must be such that the distance between two adjacent skin contacts are be equal to the two point discrimination ability of the skin at that location.
There are, in addition, requirements relating to the cross-sectional area of the skin contacts. For a given functional frequency, the minimum amplitude of vibration that the fingers can sense increases with a decrease in the cross-sectional area of the contact. A decrease in the contact area by about 1000 times (from 5.1 cm.sup.2 to 0.005 cm.sup.2) increases the minimum sensible amplitude by 30 times (from -15 dB to +15 dB). Experimental results indicate that the displacement versus contact area curve seems to level off below 0.02 cm.sup.2 and above 8 cm.sup.2. Consequently, there is no advantage in increasing the contact area above 8 cm.sup.2 or decreasing it below 0.02 cm.sup.2. Within these limits, the variation in displacement is on the order of microns only and hence choosing the cross-sectional area must be constrained by the necessary displacement.
The minimum amplitude sensible is a function of both the contact's cross-sectional area and the frequency. For a smaller amplitude, larger cross-sectional areas are preferred. However, the two point discrimination ability of the finger places an upper limit on the contact's area. Once the cross-sectional area is chosen, the amplitude is only a function of the frequency of vibration. The amplitude for a given area has a minimum between 200 Hz and 300 Hz (about 250 Hz). Thus, operating around 250 Hz would require minimum displacement (or amplitude vibration) to provide the necessary stimulation to the skin of the human fingers.
The present invention overcomes the problems associated with the tactile feedback systems previously described. The present invention is therefore directed to the problem of developing a vibro-tactile feedback system for providing the user the sensation of touch in a synthetic or virtual reality environment that is simple to manufacture and conforms to the contour of one's skin, yet affords the user a realistic sense of touch.